With respect to the energy which is used in various forms, due to concerns over depletion of petroleum resources, the search for alternative fuels and resource saving have become important issues. Under these circumstances, for fuel cells that convert various fuels to chemical energy, which is taken as electric power, active development continues.
Fuel cells are divided into the four categories of phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and polymer electrolyte fuel cells (PEFC), in accordance with the type of electrolyte used, as disclosed, for example, on page 5 of “Technology trends survey on fuel cell” (hereinafter referred to as Non-patent literature 1). These various fuel cells are restricted in their operating temperature ranges depending on the electrolyte used, and are known to operate in a low temperature range of 100° C. or lower for PEFC, in a middle temperature range of 180 to 210° C. for PAFC, in a range of 600° C. or higher for MCFC, and in a high temperature range of around 1000° C. for SOFC. Among these fuel cells, in general PEFCs capable of output in a low temperature range, electric power generated along with a chemical reaction of a hydrogen gas as a fuel with an oxygen gas (or air) is taken out. Since electric power can be efficiently taken out by a relatively small device configuration, practical applications, such as home applications or automotive applications, have been advanced.
FIG. 1 is a schematic cross-sectional view of the main part of a fuel cell, showing a basic configuration of a conventionally known PEFC. In the FIGURE, the same hatching is used to indicate constitutional components having the substantially same structure or function as materials. As shown in FIG. 1, the PEFC has a multi-layered structure of cell units, in which a membrane-electrode assembly (MEA) containing a fuel electrode (gas diffusion electrode) 17a, polymer electrolyte membrane 19, and an air electrode (gas diffusion electrode) 17c is sandwiched between a pair of bipolar plates 11a, 11c. The fuel electrode 17a contains a catalyst layer 15a, which decomposes a fuel gas into protons and electrons, and a gas diffusion layer 13a, which supplies the fuel gas to the catalyst layer 15a, and a water control layer 14a is arranged between the catalyst layer 15a and the gas diffusion layer 13a. On the other hand, the air electrode 17c contains a catalyst layer 15c, in which protons, electrons, and an oxygen-containing gas are reacted, and a gas diffusion layer 13c, which supplies the oxygen-containing gas to the catalyst layer 15c, and a water control layer 14c is arranged between the catalyst layer 15c and the gas diffusion layer 13c. 
Since the bipolar plate 11a has grooves capable of supplying the fuel gas, when the fuel gas is supplied through the grooves of the bipolar plate 11a, the fuel gas diffuses into the gas diffusion layer 13a, passes through the water control layer 14a, and is supplied to the catalyst layer 15a. The fuel gas supplied is decomposed into protons and electrons, and the protons pass through the polymer electrolyte membrane 19, and reach the catalyst layer 15c. On the other hand, the electrons pass through an external circuit (not shown), and moves to the air electrode 17c. Since the bipolar plate 11c has grooves capable of supplying the oxygen-containing gas, when the oxygen-containing gas is supplied through the grooves of the bipolar plate 11c, the oxygen-containing gas diffuses into the gas diffusion layer 13c, passes through the water control layer 14c, and is supplied to the catalyst layer 15c. The oxygen-containing gas supplied reacts with the protons, which have passed through the polymer electrolyte membrane 19, and the electrons, which have passed through the external circuit, to generate water. The generated water passes through the water control layer 14c, and is discharged to the outside of the fuel cell. On the other hand, in the fuel electrode, water reversely-diffused from the air electrode passes through the water control layer 14a, and is discharged to the outside of the fuel cell.
Functions required in the gas diffusion layer 13a and the water control layer 14a, or the gas diffusion layer 13c and the water control layer 14c include moisture retaining properties, in order to keep the polymer electrolyte membrane 19 wet under low humidity conditions, drainability, in order to avoid flooding caused by water accumulated in the fuel cell under high humidity conditions, and the like. Conventionally, with respect to the gas diffusion layer 13a and the water control layer 14a, or the gas diffusion layer 13c and the water control layer 14c, a conductive porous substrate such as carbon paper was immersed in a fluorine-based resin such as polytetrafluoroethylene, or was applied with a paste prepared by mixing carbon powder with a fluorine-based resin, to form the water control layers 14a, 14c, in which the fluorine-based resin was present, or in which the carbon powder and the fluorine-based resin were present, and to regard the portions where they were not present as the gas diffusion layers 13a, 13c. However, in the water control layers 14a, 14c formed as described above, although the conductive porous substrate was applied with the fluorine-based resin, or the carbon powder and the fluorine-based resin, the carbon paper or the like was used as the conductive porous substrate, and carbon fibers constituting the carbon paper had a high rigidity, and therefore, the carbon fibers sometimes penetrated the water control layers 14a, 14c and the catalyst layers 15a, 15c, and a short circuit sometimes occurred due to the damage of the polymer electrolyte membrane.
The applicant of the present application proposed “a gas diffusion electrode obtained by: preparing a base material for a gas diffusion electrode composed of a glass nonwoven fabric, in which a binder containing an acrylic resin and/or a vinyl acetate resin is adhered to glass fibers; and coating the base material for a gas diffusion electrode with a conductive paste containing carbon black and a polytetrafluoroethylene resin or a polyvinylidene fluoride resin, and sintering it” (Patent literature 1). However, since the glass fibers had a high rigidity, as similar to the conventional carbon paper, the glass fibers sometimes penetrated the water control layers 14a, 14c and the catalyst layers 15a, 15c, and a short circuit sometimes occurred due to the damage of the polymer electrolyte membrane.
Therefore, the present applicant further proposed “a base material for a gas diffusion electrode, comprising a nonwoven fabric containing conductive fibers containing conductive particles at least in the inside of an organic resin” (Patent literature 2). Since this base material for a gas diffusion electrode was based on an organic resin, it was flexible, and as a result, there was no case where the conductive fibers directly damaged the polymer electrolyte membrane, and a short circuit occurred. However, since a gas diffusion electrode using the base material for a gas diffusion electrode was flexible, an effect of inhibiting swelling and shrinkage of the polymer electrolyte membrane was insufficient. That is to say, the polymer electrolyte membrane repeats swelling and shrinkage depending on the humidity state during the power generation of the fuel cell. The swelling and shrinkage generated a stress between the polymer electrolyte membrane and the gas diffusion electrodes, caused by the difference between the amount of swelling and the amount of shrinkage, and as a result, distortion sometimes occurred, and eventually cracking sometimes occurred.